Use of light emitting chemical reactions for control of semiconductor production processes

ABSTRACT

A low-pressure processing vessel ( 3 ) used for the production of semiconductors is provided with a light-sensitive detector ( 1 ) at a distance from the reaction site, for example in the exhaust line ( 2 ). The detector ( 1 ) is used to detect light emitted by an intermediate which relaxes or recombines with the emission of light at a characteristic wavelength, the intermediate having a long lifetime such that the detector ( 1 ) can be positioned in a relatively remote location.

This invention relates to the control of processes used in theproduction of semiconductors, particularly but not exclusively forendpointing semiconductor etch processes, or for endpointing depositionmachine clean up processes.

Various proposals have been made for methods of predicting end point byoptical techniques. These suffer from the difficulty that complexoptical phenomena occur within the process chamber as the production orcleanup process proceeds.

An object of the present invention is to provide a method and apparatuswhich can be used in an improved form of process control.

The invention makes use of the fact that many chemical reactions—inparticular those involving free radicals and ions produced within aplasma—proceed in a number of stages through a series of intermediatesbefore a stable state or compound is produced. These intermediate stagescan have lifetimes from a few nanoseconds to a few milliseconds.Transition from one stage to another can involve the emission of light.If the chemistry of the reaction is studied and understood, and a longlifetime intermediate is identified which relaxes or recombines with theemission of light at a characteristic wavelength, then a detector tunedto that wavelength can be placed at a convenient location outside andremote from the immediate site of reaction and its output used tomonitor concentration of a species as a surrogate for material etchrate, and thus the progress of the clean-up of a vacuum processingsystem or alternatively the progress of a fabrication process thatutilizes dry-etch.

Accordingly, the present invention provides a method of controlling achemical process which takes place within a low-pressure enclosure, theprocess being such as to produce a species that emits photons of a knownwavelength or wavelength distribution by a particular chemicalrecombination or relaxation process, the species having a lifetimecharacteristic which, at the pressure of said enclosure, enables it tobe detected at a significant distance from the site of the primaryreaction, the method comprising detecting said photons at said distancewhile rejecting other photons, and using the rate at which said photonsare detected to control the process.

The term “significant distance” is used herein to mean a distance whichis significant in relation to the size of the area within which theprimary reaction occurs, and will typically be greater than 5 cm, andpreferably is of the order of 0.5 m or more.

The method may be used particularly to control the processing of siliconwith fluorine radicals, the chemical relaxation process being thecombination of the silicon difluoride radical with the fluorine radicalto yield electronically excited silicon trifluoride radical whichsubsequently returns to the ground state with the emission of a photonmost probably between 380 and 650 nm.

The silicon process will most typically be dry etching ofsilicon/silicon dioxide, or the clean-up of silicon deposited on thewalls of the enclosure during other processing. It is to be understoodthat “Silicon” includes Silicon Dioxide or other Silicon based deposits.The clean-up process may be one involving plasma enhanced chemical vaporetch, the plasma typically being produced within the enclosure.

The radicals may suitably be created upstream of the enclosure.

The photon detection may advantageously be carried out in an exhaustline from the enclosure, or in a vacuum pump to which the exhaust lineis connected.

From another aspect, the present invention provides apparatus for use inconjunction with a low-pressure enclosure serving as a reaction chamberin which takes place a chemical process which is such as to producesspecies that emits photons of a known wavelength or wavelengthdistribution by a particular chemical recombination or relaxationprocess, the species having a lifetime characteristic which, at thepressure of said enclosure, enables it to be detected at a significantdistance from the site of the primary reaction, the apparatus comprisinga photon detector arranged at a significant distance from the primaryreaction site, and means for monitoring the rate of photon detection.

Further according to the present invention there is provided apparatusfor chemical processing, comprising a low-pressure chamber and anexhaust line extending from the chamber to a vacuum pump; the chamberdefining a location in which, in use, a chemical process takes placewhich is such as to produce a species that emits photons of a knownwavelength or wavelength distribution by a particular chemicalrecombination or relaxation process, the species having a lifetimecharacteristic which, at the pressure of said enclosure, enables it tobe detected at a significant distance from the site of the primaryreaction; the apparatus further comprising a photon detector arranged ata significant distance from said location, and means for monitoring therate of photon detection.

The photon detector is preferably situated in an exhaust line of theenclosure, or in a vacuum pump to which the exhaust line is connected.

Preferably, the photon detector is provided with a light baffle toeliminate off-axis light and/or a light trap opposed to the entrance tothe detector.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the drawings, in which:

FIG. 1 is a schematic cross-sectional view of a chemical process systemused in one form of the invention;

FIG. 2 shows part of the apparatus of FIG. 1 in greater detail;

FIG. 3 is a graph showing one example of an output signal; and

FIG. 4 is a view similar to FIG. 1 of a second embodiment.

Before turning to the Figures, the physical basis underlying theembodiments will be described in more detail.

Many chemical reactions proceed through a number of intermediates. Forexample consider the following reaction of the material M with the freeradical R:M(s)+nR

MR_(n)(g)   a)MR_(n)+R

MR_(n+1)*(g)   b)MR_(n+1)*(g)

MR_(n+1)(g)+photon   c)MR_(n+1)(g)+R

MR_(n+2)(g)   d)

In reaction a) the material is removed from the solid surface into thegas phase by combination with one or more individuals of the radicalspecies. In reaction b) further stepwise combination yields anelectronically energetic intermediate which decays to the ground statewith the release of a photon. Further stepwise addition gives the finalstable product which is pumped from the system.

In general, in the absence of quenching agents, the production of lightat step c) will be directly proportional to the concentration of theelectronically exited intermediate and can be used as an indicator thatthe reaction removing (etching) material M is proceeding.

In those particular circumstances where reaction b) is rate limiting theoverall reaction cascade, and where the intermediate is sufficientlystable that b) is the predominant reaction, then the production of lightat step c), which is necessarily a very fast process, will be aquantitative surrogate for the rate of removal of the material M. Evenwhere reaction b) is not rate limiting the presence or absence of theproduction of light can be used as an endpoint indication for theremoval of material M.

If the process pressure and the concentration of radical R arefavorable, the emission of light associated with the etch process willbe located remotely from the surface of the material M and may extendsome metres into the surrounding space.

Given that the normal operating pressures in vacuum processing systemsduring the clean-up, preventative maintenance phase, or alternativelythat during device fabrication by dry etch, is in the low mtorr range,the mean free path of the intermediates will be high compared with thegeometries of typical process equipment. Given also that at suchpressures the concentration of the reactants are necessarily low therebyenhancing the halflife of the intermediate MR_(n), light emission due tostep c) will occur as much as 1 metre away from the vacuum processingsystem's surfaces which are the subject of the cleaning process itself,or alternatively from the material being fabricated during a dry etchprocess. This distance is simply derived from the halflife of thespecies MR_(n) and its diffusion rate.

Since monitoring of the emitted light is carried out remotely from thereaction region it can be done with no interference or disruption to thereaction itself. For example, without prejudice to the generality of thetechnique, it may be possible to site the measurement means convenientlyon the exhaust line from the processing vessel.

Without prejudice to the generality of the technique, one example of auseful reaction which is typical of the type of reaction describedabove, is the etching of silicon by the fluorine radical generated byplasma decomposition of compounds including, but not restricted to theperfluoronated hydrocarbons, sulphur hexafluoride and nitrogentrifluoride.Si+2F

SiF₂   a)SiF₂+F

SiF₃*   b)SiF₃*

SiF₃+photon   c)SiF₃*+F

SiF₄   d)

In a typical silicon etch process, where base pressures are in the range1 to 100 mtorr, the silicon difluoride radical produced by reaction a)has a lifetime of several milliseconds, and can diffuse a considerabledistance before conversion to the trifluoride radical takes place withthe immediate relaxation of the excited state and comcomitant productionof a photon.

The light emitted is a quasi-continuum ranging from 380 to 650 nm. Giventhe geometry of a typical process chamber, a substantial amount of thislight will be emitted in the exhaust line.

A first embodiment of the invention will now be described with referenceto FIGS. 1 to 3.

FIG. 1 shows a typical vacuum processing vessel 3 the diameter of whichis of the order of 1 metre. The processing vessel 3 is maintained at alow pressure by a vacuum pump 11 via an exhaust line 2.

A typical vacuum processing technique which is the basic function of thevessel would be the deposition of polysilicon by introduction into thevessel a compound such as, but not restricted to, an organo-siliconcompound, and then dissociating that compound at the heated substratesurface 5. A side effect of this procedure is to deposit silicon on thewalls of the process vessel 3. It is to be understood that “Silicon”includes Silicon Dioxide or other silicon based deposits. Such siliconis a disadvantage to the basic function of the vessel as it contributesto particulates and consequential failure of devices. A typical clean-uptechnique would be the introduction of nitrogen trifluoride into anup-stream plasma region 4 where it is dissociated to yield the freefluorine radical. The fluorine radical reacts with the silicon which hasbeen deposited on the walls of the vessel, and follows the reactionsequence already outlined above for silicon. The pressure in thereaction vessel during the clean up process will be of the order of 1 to100 mtorr.

In order to maximize the efficiency of the clean up process it isdesirable to monitor the rate of etch of the silicon contaminate andthen stop the clean up process once it has dropped below apre-determined level.

In FIG. 1 a detector 1 is shown connected to the exhaust line 2 of theprocess vessel 3. FIG. 2 shows a detail of one suitable form of detectorwhich in this case consists of a light-baffle 7 in front of a wavelengthdiscriminating filter 8 and a photomultiplier tube 9. Opposite thedetector 1 is placed a light-trap 6. The light-baffle 7 in this exampleconsists of a number of opaque parallel plates in front of the filter 8,each perforated with a multiplicity of apertures, the pattern andspacial arrangement of which has the function of rejecting light whichmay be reflected from the reaction region along the exhaust line 2. Thelight-trap 6 comprises an open-ended cylinder whose walls are providedwith projecting baffles, all the internal surfaces being matt black formaximum light absorption.

In a specific embodiment, the light-baffle 7 would consist of a seriesof opaque plates with apertures in the plates arranged one after theother so that only on-axis light can reach the photon detector.

The apertures in the plates are arranged such that their distributionpattern in the direction in the plane of the individual plates isaperiodic so as to avoid the situation where off-axis light could passthrough one aperture in the first plate at a particular angle so that amultiple rule would allow it to pass through not the associatedline-of-sight aperture in the next plate but one off-axis and, by virtueof the same multiple rule then pass through other line-of-sightapertures in subsequent plates. The size of the apertures are arrangedto increase from one plate to another so that the smallest sizedapertures are in the plate adjacent to the photon detector and thelargest sized apertures are in the plate furthest away with the increasein size arranged in such a way that an observer located at the photondetector would only be able to see the edges of the apertures in theplate adjacent to him and not be able to view any of the edges in theapertures in plates not adjacent to him.

The combination of the light-baffle 7 and the opposed light-trap 6 hasthe effect that only light emitted within the tightly confined detectionregion is detected.

FIG. 3 shows a typical output from the photomultiplier in graphical formwhich may be used to determine the rate of removal of siliconcontaminant. By reference to historical cleaning cycle data, digitalsignal process techniques such as, but not restricted to, curve shaperecognition and perturbation analysis can be used to yield an endpointdecision which can then used to automate the clean-up cycle.

With reference to FIG. 4, a second embodiment is used for controllingthe etching of silicon.

FIG. 4 shows a typical vacuum processing vessel 3 the diameter which isof the order of 1 metre. A typical processing technique would be theetching of silicon by introduction into the vessel a fluorine sourcesuch as, but not restricted to, sulphur hexafluoride as a gas. Thepressure in the reaction vessel will be in the range 1 to 100 mtorr andan electric field is applied to two electrodes 13 in such a manner thata plasma is formed between them. The silicon substrate 14 to be etchedis placed on the ground connected electrode and the fluorine radicalproduced by the dissociation of the sulphur hexafluoride reagent reactswith the silicon according to the reaction sequence already outlinedabove.

In this example, consider that a layer of silicon dioxide exists withinthe silicon wafer 14 and it is desired to etch down to this buriedlayer, and stop at it in a timeous manner. Once the etch of silicon hasreached the silicon dioxide layer, the etch rate decreases and thisevent may be detected by the reduction in concentration of siliconreaction intermediates.

In FIG. 4, as in FIG. 1, a detector 1 is connected to the exhaust line 2of the process chamber 3. The detector 1 may suitably be the same as isshown in FIG. 2, and the output from the detector will be of the sameform as is shown in FIG. 3.

Modifications may be made to the foregoing embodiments within the scopeof the invention as defined in the claims. The detector could be placedwithin the processing vessel itself at a suitable distance form thereaction site, or could be incorporated in the vacuum pump; however,positioning the detector in communication with the exhaust line islikely to be the most convenient arrangement. The invention may be usedwith processes other than silicon/fluorine wherever a suitablelight-emitting intermediate stage is present.

1. A method of controlling a chemical process which takes place within alow-pressure enclosure, the method comprising conducting a chemicalprocess which produces a species that emits photons of a knownwavelength or wavelength distribution by a particular chemicalrecombination or relaxation process, the species having a lifetimecharacteristic which, at the pressure of said enclosure, enables it tobe detected at a significant distance from the site of the primaryreaction, the method further comprising detecting said photons at saiddistance while rejecting other photons, and using the rate at which saidphotons are detected to control the process.
 2. A method according toclaim 1, in which said significant distance is greater than 5 cm.
 3. Amethod according to claim 2, in which said significant distance is ofthe order of 0.5 m or more.
 4. A method according to claim 1, in whichthe chemical process comprises the processing of silicon with fluorineradicals, the chemical relaxation process being the combination of thesilicon difluoride radical with the fluorine radical to yieldelectronically excited silicon trifluoride radical which subsequentlyreturns to the ground state with the emission of a photon most probablybetween 380 and 650 nm.
 5. The method of claim 4, in which the siliconprocess is dry etching of silicon/silicon dioxide.
 6. The method ofclaim 4, in which the silicon process is the clean-up of silicon/SiliconDioxide or other silicon based material deposited on the walls of theenclosure during other processing.
 7. The method of claim 6, in whichthe clean-up process makes use of plasma enhanced chemical vapor etch,the plasma being produced within the enclosure.
 8. The method of claim7, in which the plasma is produced from radicals created upstream of theenclosure.
 9. A method according to claim 1, in which the photondetection is carried out in an exhaust line from the enclosure, or in avacuum pump to which the exhaust line is connected.
 10. Apparatus foruse in conjunction with a low-pressure enclosure serving as a reactionchamber in which takes place a chemical process which is such as toproduce a species which emits photons of a known wavelength orwavelength distribution by a particular chemical recombination orrelaxation process, the species having a lifetime characteristic which,at the pressure of said enclosure, enables it to be detected at asignificant distance from the site of the primary reaction, theapparatus comprising a photon detector arranged at a significantdistance from the primary reaction site, and means for monitoring therate of photon detection.
 11. Apparatus for chemical processing,comprising a low-pressure chamber and an exhaust line extending from thechamber to a vacuum pump; the chamber defining a location in which, inuse, a chemical process takes place which is such as to produce aspecies that emits photons of a known wavelength or wavelengthdistribution by a particular chemical recombination or relaxationprocess, the species having a lifetime characteristic which, at thepressure of said enclosure, enables it to be detected at a significantdistance from the site of the primary reaction; the apparatus furthercomprising a photon detector arranged at a significant distance fromsaid location, and means for monitoring the rate of photon detection.12. Apparatus according to claim 11, in which the photon detector issituated in the exhaust line or in the vacuum pump to which the exhaustline is it connected.
 13. Apparatus according to claim 11, in which thephoton detector is provided with a light baffle to eliminate off-axislight and/or a light trap opposed to the entrance to the detector. 14.Apparatus according to claim 13, in which the light baffle includes anumber of plates having apertures which are arranged aperiodically. 15.Apparatus according to claim 14, in which the size of the apertures arearranged to increase from one plate to another such that the aperturesof smallest size are in the plate adjacent the photon detector and theapertures of largest size are in the plate furthest from the photondetector.
 16. Apparatus according to claim 15, in which the sizes andarrangement of the apertures are such that an observer located at thephoton detector can only see edges of the apertures in the plateadjacent to him whilst being unable to view any edges in the aperturesin the plates which are not adjacent to him.
 17. Apparatus according toclaim 11, in which the photon detector is spaced at least 5 cm from saidlocation.
 18. Apparatus according to claim 17, in which the photondetector is spaced 0.5 m or more from said location.